Earthquakes, tsunamis, mud-rock flows and volcanoes have been great threats to all human beings. It has been reported that during the pre-earthquakes period, the local geomagnetic field, as well as their quiet-solar daily variations, are deviated from their normal values. This calls for continuous surveillance of the geomagnetic field via a distributed network consisting of variance of platforms. The magnetic field vector, acquainted on the airborne platforms, is required to be expressed in a unified geographic coordinate frame. However, the attitude measurements are noise-contaminated, taking into account that the ambient magnetic field is numerically of large scale, ranging from over 60,000 nT in the south or north pole to approximately 20,000 nT over the equator. The transformation of the magnetic field from the body frame to the geographic frame results in unacceptable errors, which may cause failure to detect the anomalies of measurement.
For several decades, aeromagnetic survey has been widely used in fields such as mineral, petroleum and geothermal exploration, and submarine detection for military proposes, all in a platform-centered manner, where only the magnetic anomalies are of interest. Most aeromagnetic survey systems acquire total-field magnetic data only. However, total-field-only survey is not sufficient for the purpose of pre-earthquake magnetic field surveillance.
The followings are further relevant examples that address the issue of aeromagnetic survey and aeromagnetic data processing.
1. U.S. Pat. No. 5,355,313 granted to Moll et al uses neural networks to determine depth of basement rock.
2. U.S. Pat. No. 5,884,256 granted to Moll et al discloses the data processing system that utilizes neural networks to determine mine depth of basement rock, and in particular, describes the layout and working process of the neural network. The above two patents do not describe the measurement and transformation of magnetic field.
3. U.S. Pat. No. 6,021,577 granted to Shiells et al discloses a technique that is utilized to determine a mine orientation by a base station and a mobile site, both made of two or three mutually orthogonal fluxgates magnetometers.
4. The technique report, entitled IMPAC Integrated Multi-Parameter Airborne Console, available at the website www.Picoenvirotec.com, discloses an airborne magnetic field survey system that is provided with four optically pumped cesium vapor magnetometers and a tri-axial fluxgate magnetometer and used to offer the attitude of the aircraft and generate the compensation models of magnetic environment. However, it is not mentioned how the measuring outputs of tri-axial fluxgate magnetometer are transformed into a geographic coordinate frame.
The electromagnetic environment and magnetic field daily variation caused by solar radiation are two important impacts that keep the airborne magnetometers from sensing correct geomagnetic field, and need to be compensated for. However, this invention assumes that the vehicle's magnetic environment and the solar daily variation have been properly compensated, and concentrated on the issue of reduction of measuring errors of the magnetic field due to the inaccurate attitude measurements. The installation error and equipment error are also assumed to be corrected properly.
In order to reduce the magnetic field error induced by inaccurate coordinate transformation, an approach is one of employing three or more twin-frequency GPS receivers with their antennas non-collinear assembly over the aircraft; this approach ensures that the baselines, connecting the antennas, are long sufficiently.
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